Polymer adsorption onto random planar surfaces: Interplay of polymer and surface correlations

Tailoring Interactions of Random Copolymer Polyelectrolyte Complexes to Remove Nanoplastic Contaminants from Water

We investigate the usage of polyelectrolyte complex materials for water remediation purposes, specifically their ability to remove nanoplastics from water, on which there is currently little to no prior research. We demonstrate that oppositely charged random copolymers are effective at quantitatively removing nanoplastic contamination from aqueous solution. The mechanisms underlying this remediation ability are explored through computational simulations, with corroborating quartz crystal microbalance adsorption experiments. We find that hydrophobic nanostructures and interactions likely play an important role.

Random copolymer adsorption onto a periodic heterogeneous surface: A partially directed walk model

The adsorption of a single AB random copolymer (RC) chain onto an inhomogeneous ab surface with a regular periodic pattern is studied theoretically. The problem is considered within the simplest model of a partially directed random walk in two dimensions by using the method of generating functions and the annealed approximation for the averaging over disorder in the RC sequence. The existence of the "optimal" RC composition and the degree of correlation in the monomer sequence, at which the inverse transition temperature has a local minimum, is shown. This is characteristic for symmetric and weakly asymmetric surfaces, whereas for surfaces with pronounced asymmetry there is no such local minimum. The best adsorbate for a strongly asymmetric surface is the homopolymer composed of monomer units that are complimentary to the majority sites on the surface. The results for the adsorption transition point obtained in the annealed approximation are compared with the numerical results for random-periodic AB-copolymers with a long period being a quenched random sequence of A and B units. The comparison shows that the annealed approximation provides a very good quantitative estimate of the adsorption transition point.

Adsorption and Desorption of Polymers on Bioinspired Chemically Structured Substrates

Natural biological surfaces exhibit interesting properties due to their inhomogeneous chemical and physical structure at the micro- and nanoscale. In the case of hair or skin, this also influences how waterborne macromolecules ingredients will adsorb and form cosmetically performing deposits (i.e., shampoos, cleansers, etc.). Here, we study the adsorption of hydrophilic flexible homopolymers on heterogeneous, chemically patterned substrates that represent the surface of the hair by employing coarse-grained molecular dynamics simulations. We develop a method in which the experimental images of the substrate are used to obtain information about the surface properties. We investigate the polymer adsorption as a function of polymer chain length and polymer concentration spanning both dilute and semidilute regimes. Adsorbed structures are quantified in terms of trains, loops, and tails. We show that upon increasing polymer concentration, the length of tails and loops increases at the cost of monomers belonging to trains. Furthermore, using an effective description, we probe the stability of the resulting adsorbed structures under a linear shear flow. Our work is a first step toward developing models of complex macromolecules interacting with realistic biological surfaces, as needed for the development of more ecofriendly industrial products.

Heterogeneous Charged Complexes of Random Copolymers for the Segregation of Organic Molecules

Nature harnesses the disorder of intrinsically disordered proteins to organize enzymes and biopolymers into membraneless organelles. The heterogeneous nature of synthetic random copolymers with charged, polar, and hydrophobic groups has been exploited to mimic intrinsically disordered proteins, forming complexes with enzymatically active proteins and delivering them into nonbiological environments. Here, the properties of polyelectrolyte complexes composed of two random copolymer polyelectrolytes are studied experimentally and via simulation with the aim of exploiting such complexes for segregating organic molecules from water. The anionic polyelectrolyte contains hydrophilic and hydrophobic side chains and forms self-assembled hydrophobic domains. The cationic polymer is a high-molecular-weight copolymer of hydrophilic and charged side groups and acts as a flocculant. We find that the polyelectrolyte complexes obtained with this anionic and cationic random copolymer system are capable of absorbing small cationic, anionic, and hydrophobic organic molecules, including perfluorooctanoic acid, a compound of great environmental and toxicologic concern. Importantly, these macroscopic complexes can be easily removed from water, thereby providing a simple approach for organic contaminant removal in aqueous media. MARTINI and coarse-grained molecular dynamics simulations explore how the microscale heterogeneity of these random copolymer complexes relates to their segregation functionality.

Adsorption of amphiphilic graft copolymers in solvents selective for the grafts on a lyophobic surface: a coarse-grained simulation study

The sorption of graft copolymers on surfaces attractive only for the backbone and its effect on the conformational behavior of adsorbed/desorbed chains in solvents good for the grafts and poor for the backbone was studied by coarse-grained computer simulations.

Dry and wet interfaces: influence of solvent particles on molecular recognition

We present a coarse-grained lattice model to study the influence of water on the recognition process of two rigid proteins. The basic model is formulated in terms of the hydrophobic effect. We then investigate several modifications of our basic model showing that the selectivity of the recognition process can be enhanced by considering the explicit influence of single solvent particles. When the number of cavities at the interface of a protein-protein complex is fixed as an intrinsic geometric constraint, there typically exists a characteristic fraction that should be filled with water molecules such that the selectivity exhibits a maximum. In addition, the optimum fraction depends on the hydrophobicity of the interface so that one has to distinguish between dry and wet interfaces.

Influence of correlations on molecular recognition

The influence of the patchiness and correlations in the distribution of hydrophobic and polar residues at the interface between two rigid biomolecules on their recognition ability is investigated in idealized coarse-grained lattice models. A general two-stage approach is utilized where an ensemble of probe molecules is designed first and the recognition ability of the probe ensemble is related to the free energy of association with both the target molecule and a different rival molecule in a second step. The influence of correlation effects are investigated using numerical Monte Carlo techniques and mean field methods. Correlations lead to different optimum characteristic lengths of the hydrophobic and polar patches for the mutual design of the two biomolecules on the one hand and their recognition ability in the presence of other molecules on the other hand.

Coarse-grained lattice model for investigating the role of cooperativity in molecular recognition

Equilibrium aspects of the molecular recognition of rigid biomolecules are investigated using coarse-grained lattice models. The analysis is carried out in two stages. First, an ensemble of probe molecules is designed with respect to the target biomolecule. The recognition ability of the probe ensemble is then investigated by calculating the free energy of association. The influence of cooperative and anticooperative effects accompanying the association of the target and probe molecules is studied. Numerical findings are presented and compared to analytical results which can be obtained in the limit of dominating cooperativity and in the mean-field formulation of the models.

Developing and analyzing idealized models for molecular recognition

We study equilibrium aspects of molecular recognition of two biomolecules using idealized model systems and methods from statistical physics. Starting from the basic experimental findings we demonstrate exemplarily how an idealized coarse-grained model for the investigation of molecular recognition of two biomolecules can be developed. In addition we provide details regarding two model systems for the recognition of a flexible and a rigid biomolecule respectively, the latter taking into account conformational changes. We focus particularly on the interplay and influence of the correlations of the residue distributions of the biomolecules on the recognition process.

Separation of DNA with different configurations on flat and nanopatterned surfaces

We demonstrate that electrophoresis on a flat Si substrate is an effective method in separation of DNA with different configurations, e.g., linear, supercoiled, and relaxed or DNA of different length, e.g., supercoiled DNA ladder. The surface separation arises from the different number of contacts due to the conformational differences between adsorbed DNA chains. Imposing a Au nanopattern on the Si surface further improves the separation effect. The simulation of electric field on this patterned surface by the finite element method shows that Au nanodots act as local pinning points for DNA segments due to dielectrophoretic force. The results of molecular dynamics simulation showed that the conformational differences between adsorbed polymer chains were amplified on the patterned surface and enhanced separations were achieved, which are consistent with the experimental results.

Adsorption of diblock copolymers on stripe-patterned surfaces

We present the results of extensive Monte Carlo simulations of diblock copolymers adsorbed on stripe-patterned surfaces of various widths. We have found that the width of the stripe pattern is an important parameter which dictates favorable recognition on the surface. For certain stripe widths, the adsorption of diblock copolymers to striped surfaces exhibits two transitions. The process involves recognition of the surface pattern by the diblock copolymer which follows a two step process in which the first block getting adsorbed to the appropriate pattern on the surface, without any recognition of the surface pattern, followed by the adsorption of the second block, where a reorganization process happens. For small widths and also for higher widths, the chain behaves just like a homopolymer where the twofold adsorbing process changes to the typical homopolymer adsorption. We have also found that there exists an optimal width of the stripes, independent of the chain length, where the recognition on the surface pattern is most favored. The characteristic temperature of the adsorption of the second block with weaker interactions is found to be independent of the chain length at this optimal width, proving that only local rearrangements take place after the first step. Some of our results describing the thermodynamics compare very well with the recent semianalytical approach of Kriksin et al. [J. Chem. Phys. 122, 114703 (2005)] on multiblock copolymers on heterogeneous surfaces. We also present some interesting conformational properties of the copolymer chain near the stripe-patterned surface.

Coarse-grained lattice model for molecular recognition

We present a simple model which allows us to investigate the equilibrium aspects of molecular recognition between rigid biomolecules on a generic level. Using a two-stage approach, which consists of a design and a testing step, the role of cooperativity and of varying bond strength in molecular recognition is investigated. Cooperativity is found to enhance selectivity. In complexes which require a high binding flexibility, a small number of strong bonds seems to be favored compared to a situation with many but weak bonds.

What really enhances the adsorption of polymers onto chemically nonuniform surfaces: Surface randomness or its heterogeneity?

We theoretically perform a comparative analysis of the adsorption of polymers onto the regularly and randomly nonuniform surfaces. By developing and making use of the self-consistent perturbation expansion we calculate the surface excesses of the polymers adsorbed on the random and periodically patterned surfaces. In both cases the enhancement of the polymer adsorption is indicated, as compared to the adsorption onto the homogeneous surface that has the same average affinity for polymers. Moreover, the results obtained for the randomly nonuniform and periodically patterned adsorbing surfaces show striking quantitative similarity, when compared at the same characteristic sizes of inhomogeneities of these surfaces. This finding leads to the conclusion that the adsorption ability of the nonuniform surface primarily depends on the characteristic size of the surface inhomogeneity, rather than on the spatial distribution of the inhomogeneities on this surface. In all cases, the calculated total surface excess is found to be a decaying function of the ratio of the radius of gyration of polymers to the characteristic size of the surface inhomogeneity. The effect of the excluded volume is found to reduce the polymer adsorption.